2. Thermo-chemistry of Engine Combustion P M V Subbarao Professor Mechanical Engineering Department A n Important Clue to Control Rate of Heat Release ….

3. Real Combustion & Model Testing

4. Results of Model Testing. For a given fuel and required Power & Speed conditions. Optimum composition of Exhaust Gas. Optimum air flow rate. Optimum fuel flow rate. Optimum combustion configuration!!! Molar Analysis of Dry Exhaust Products: Mole fraction of CO 2 : x 1 Mole fraction of CO : x 2 Mole fraction of O 2 : x 4 Mole fraction of N 2 : x 5

5. Stoichiometry of Actual Combustion C X H Y +  4.76 (X+Y/4) AIR → P CO 2 +Q H 2 O + T N 2 + U O 2 + V CO Conservation species: Conservation of Carbon: X = P+V Conservation of Hydrogen: Y = 2 Q Conservation of Oxygen : K + 2  (X+Y/2) = 2P +Q +2U+V Conservation of Nitrogen: 2  3.76 (X+Y/2+Z-K/2) = T

6. For every 100 kg of fuel. C X H Y +  4.76 (X+Y/4) AIR + Moisture in Air + Ash & Moisture in fuel → P CO 2 +Q H 2 O ++ T N 2 + U O 2 + V CO + W C + Ash

7. Dry Exhaust gases: P CO 2 + T N 2 + U O 2 + V CO kmols. Volume of gases is directly proportional to number of moles. Volume fraction = mole fraction. Volume fraction of CO 2 : x 1 = P * 100 /(P + T + U + V) Volume fraction of CO : x 2 = VCO * 100 /(P + T + U + V) Volume fraction of O 2 : x 4 = U * 100 /(P + T + U + V) Volume fraction of N 2 : x 5 = T * 100 /(P + T + U + V) These are dry gas volume fractions. Emission measurement devices indicate only Dry gas volume fractions.

8. Measurements: Volume flow rate of air. Volume flow rate of exhaust. Dry exhaust gas analysis. x 1 +x 2 +x 3 + x 4 + x 5 = 100 or 1 Ultimate analysis of coal. Combustible solid refuse. nC X H Y +  n 4.76 (X+Y/4) AIR + Moisture in Air → x 1 CO 2 +x 6 H 2 O + x 5 N 2 + x 4 O 2 + x 2 CO + x 7 C

9. nC X H Y +  n 4.76 (X+Y/4) AIR + Moisture in Air + → x 1 CO 2 +x 6 H 2 O + x 5 N 2 + x 4 O 2 + x 2 CO + x 7 C x 1, x 2,x 3, x 4 &x 5 : These are dry volume fractions or percentages. Conservation species: Conservation of Carbon: nX = x 1 +x 2 +x 7 Conservation of Hydrogen: nY = 2 x 6 Conservation of Oxygen : nK + 2 n  (X+Y/4) = 2x 1 +x 2 +2x 4 +x 6 Conservation of Nitrogen:  n 3.76 (X+Y/4+Z-K/2) = x 5

10. nC X H Y +  n 4.76 (X+Y/4) AIR + Moisture in Air → x 1 CO 2 +x 6 H 2 O + x 5 N 2 + x 4 O 2 + x 2 CO + x 7 C + Ash Re arranging the terms (Divide throughout by n): C X H Y +  4.76 (X+Y/4) AIR + Moisture in Air → (x 1 /n)CO 2 +(x 6 /n) H 2 O + (x 5 /n) N 2 + (x 4 /n) O 2 + (x 2 /n) CO + (x 7 /n) C C X H Y +  4.76 (X+Y/4) AIR + Moisture in Air → P CO 2 +Q H 2 O + T N 2 + U O 2 + V CO + W C

11. Air-fuel Ratio: Fuel Lean Mixtures :  Fuel-rich Mixtures:  >1 Equivalence ratio:

12. Partial Pressure of air in Intake System In a SI engine, the presence of gaseous fuel, moisture in the intake air and residual exhaust gases reduces the intake air partial pressure below the mixture pressure. In a CI engine, the presence of moisture in the intake air and residual exhaust gases reduce the intake air partial pressure below the mixture pressure. For a mixture:

13. Fraction of air in the Cylinder The residual gas fraction in the cylinder during compression is determined by the exhaust and inlet processes. Its magnitude affects volumetric efficiency and engine performance directly. The residual gas fraction is a function of inlet and exhaust pressures, speed, compression ratio, valve timing, and exhaust system dynamics. The residual gas fraction is defined as:

14. Residual Gas Fraction: Effect of Speed Intake

15. Residual Gas Fraction: Effect of Valve Overlap Intake

16. Residual Gas Fraction : Effect of Compression Ratio Intake

17. Actual Mass of air per Cycle : Volumetric Efficiency Volumetric efficiency a measure of overall effectiveness of engine and its intake and exhaust system as a natural breathing system. It is defined as: If the air density  a,0 is evaluated at inlet manifold conditions, the volumetric efficiency is a measure of breathing performance of the cylinder, inlet port and valve. If the air density  a,0 is evaluated at ambient conditions, the volumetric efficiency is a measure of overall intake and exhaust system and other engine features. The full load value of volumetric efficiency is a design feature of entire engine system.

18. Volumetric Efficiency of A Cycle The volumetric efficiency is a function of Intake mixture pressure p i. Intake mixture Temperature T i. Fuel/ air ratio (F/A). Compression ratio r v. Exhaust pressure, p e. Let m is the mass of gas in the cylinder at the end of intake stroke.

19. Full Load Overall Volumetric Efficiency Overall volumetric efficiency is affected by following variables. Intake and exhaust manifold and port design. Intake and exhaust valve geometry, size, lift and timings. Fuel type, fuel/air ratio, fraction of fuel vaporized in the intake system, and fuel heat vaporization. Mixture temperature as influenced by heat transfer. Ratio of exhaust to inlet manifold pressures. Compression ratio. Engine speed. The effects of many of above variables are quasi-steady in nature. Their impact is either independent of speed or adequately function of speed.

20. Anatomy of Volumetric Losses Quasi-static Effects Charge/air Heating Flow friction Choking Backflow

21. Combustion Efficiency of Engine The fraction of fuel chemical energy not available due incomplete combustion is quantified using combustion efficiency. The net chemical energy release due to actual combustion with in the engine is: The combustion Efficiency:

22. Variation of Combustion Efficiency with Equivalence Ratio

23. MATt  Theory Proposed by Dixon Mixing: Proper Mixing of fuel and air. Air: Sufficient amount of air. T : Sufficient temperatures. t : Sufficient time.  : Local density of air and fuel. A Phenomenological Theory

24. Realization of MATt  Theory Mixing: Fuel preparation systems. Air: Intake and exhaust manifolds &valves. T : Preheating of fuel through adiabatic compression. t : Duration of combustion process.  : Turbulence generation systems.

26. Care for Occurrence of Heat Addition Occurrence of Heat Addition in SI Engine : A Child Care Event. Occurrence of Heat Addition in CI Engine: A Teen Care Event. CI Engine SI Engine

27. Type of Fuel Vs Combustion Strategy Highly volatile with High self Ignition Temperature: Spark Ignition. Ignition after thorough mixing of air and fuel. Less Volatile with low self Ignition Temperature: Compression Ignition, Almost simultaneous mixing & Ignition.